[3.7] gh-96848: Fix -X int_max_str_digits option parsing (#96988)

[vstinner]

Fix command line parsing: reject "-X int_max_str_digits" option with no value (invalid) when the PYTHONINTMAXSTRDIGITS environment variable is set to a valid limit.

(cherry picked from commit 4135166)

• Issue: Minor parsing issue with new -X int_max_str_digits option if PYTHONINTMAXSTRDIGITS #96848

[vstinner]

Ah, Windows (x64) failed with a time issue:

FAIL: test_denial_of_service_prevented_str_to_int (test.test_int.IntStrDigitLimitsTests)
Regression test: ensure we fail before performing O(N**2) work.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\a\cpython\cpython\lib\test\test_int.py", line 654, in test_denial_of_service_prevented_str_to_int
    self.assertLess(seconds_to_fail_extra_huge, seconds_to_convert/8)
AssertionError: 0.015625 not less than 0.013671875

I re-run the failed job.